sdp: Negotiate extmap-allow-mixed

[haaspors]

This allows both one-byte and two-byte headers

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.50%. Comparing base (1b3e9c4) to head (d907f8f).
Report is 2 commits behind head on master.

Files	Patch %	Lines
webrtc/src/peer_connection/sdp/sdp_test.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
- Coverage   60.53%   60.50%   -0.04%     
==========================================
  Files         482      482              
  Lines       47930    47942      +12     
  Branches    12470    12485      +15     
==========================================
- Hits        29016    29007       -9     
- Misses       9669     9673       +4     
- Partials     9245     9262      +17     

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